Theoretical Study On Electrocatalysis And Environmental Stability Of Several Two-dimensional Materials

As new members of the family of nano-materials,two dimensional?2D?materials of atomic thickness,such as Graphene,MoS₂,Black Phosphorus?BP?,Boron monolayer,InSe etc.,have attracted tremendous interest in recent years,owing to their excellent physical and chemical properties and application prospect in various fields.Under this circumstance,opportunities and challenges coexist for researchers.Opportunities are that there is a huge space for performance optimization and development of new two dimensional materials.Due to their advantages of large catalytic areas,excellent mechanical properties,low-cost,new two-dimensional materials have gained great attentions,especially for energy and catalysis.However,metal-free 2D catalysts are very rare and the catalytic mechanism is not clear.Thus,it is very important to find high-efficiency metal-free 2D catalysts and study their catalytic mechanism.In case of challenges,although 2D materials own distinguished performance in various fields,they also faced many problems,such as environmental instability and performance degradation when incorporating into devices,which greatly limit their practical application.Thus,it is urgent to solve these shortcomings.In this thesis,on the basis of first-principles calculation,we designed high-performance metal-free catalyst toward hydrogen evolution reaction?HER?based on 2D boron monolayer and developed the catalyst of nitrogen fixation based 2D BP.Moreover,we also revealed the degradation mechanism of InSe and GaSe,further proposed effective protection strategy.The main conclusions are summarized below:?1?Two-dimensional boron monolayers as a high-performance catalyst for hydrogen evolution reaction.Two-dimensional?2D?boron monolayers have been successfully synthesized on a silver substrate very recently.Their potential application is thus of great significance.We explore the possibility of boron monolayers?BMs?as electrocatalysts for the hydrogen evolution reaction?HER?by first-principles methods.Our calculations show that BMs are active catalysts for HER with free energy of hydrogen adsorption close to zero,metallic conductivity and plenty of active sites in the basal plane.The effect of the substrate on HER activity is further assessed.It is found that the substrate has a positive effect on the HER performance caused by the competitive effect of mismatch strain and charge transfer.The in-depth understanding of the structure dependent HER activity is also provided.?2?Metal-free electrocatalyst for reducing nitrogen to ammonia using a Lewis acid pair.Electrocatalytic nitrogen reduction reaction?NRR?is a promising way for the sustainable production of ammonia.Herein,on the basis of the concept of“Lewis acid pair”,we propose a metal-free electrocatalyst based on boron-decorated black phosphorus?BP?.In the integrated structure of the catalyst,the doped B atoms serve as Lewis acid pairs and catalytic centers,while the channels of BP provide structural advantages for hosting the pair and activating the N?N bond.A new strategy of nitrogen activation based on the pull–pull effect is thus developed.Performance evaluations show that this metal-free catalyst is highly efficient for electrocatalytic nitrogen reduction with an ultra-low onset-potential of 0.19 V.This work opens a new possible avenue for nitrogen activation and can be generally applied to other two-dimensional or bulk materials.?3?Oxidation mechanism and protection strategy of ultrathin InSe:insight from theory.Ultrathin indium selenide?InSe?,as a newly emerging two dimensional material with unique electronic properties,has been the focus of current research.However,the long-term environmental instability of atomically thin InSe seriously limits its practical applications.By employing density functional theory and ab initio molecular dynamics simulations,we provide an in-depth understanding of the oxidation mechanism of InSe.The defect-free InSe presents excellent stability against oxidation.Nevertheless,the Se vacancies on the surface can react with water and oxygen in air directly and activate the neighboring In-Se bonds on the basal plane for further oxidation,leading to complete degradation of InSe into oxidation products of In₂O₃ and elemental Se.Furthermore,we propose an efficient strategy to repair the Se vacancies by thiol chemistry.In this way,the repaired surface can resist oxidation from oxygen and retain the original high electron mobility of pristine InSe simultaneously.?4?Effect of illumination and Se vacancies on fast oxidation of ultrathin GaSe.Compared to the same group of InSe,exposure for a few days causes the fast oxidation of ultrathin GaSe under ambient conditions and the oxidation mechanism remains unclear.By means of density functional theory calculations and ab initio molecular dynamics simulations,our results show that illumination and Se vacancies induce the fast oxidation of GaSe.Under illumination,photo-excited electrons from the surface of GaSe are effectively transferred to oxygen molecules and thus,superoxide anions?O₂^-?are generated that react with GaSe.Moreover,Se vacancies directly react with O₂.In both the cases,the Ga-Se bonds are continually replaced by Ga-O bonds,which eventually leads to complete degradation of GaSe,accompanied with the formation of the oxidation products Ga₂O₃ and elemental Se.The comprehensive degradation mechanism unveiled herein lays an important foundation for the development of suitable protecting strategies in GaSe-based devices.